This invention relates to improved sulphur recovery units and to processes which use these units. More particularly, this invention relates to a sulphur recovery process and apparatus which continuously circulates catalyst and which may be operated in a smooth and continuous fashion.
The present invention is directed to chemical reactions leading to the removal of sulphur compounds from gaseous streams. This removal is generally carried out through the partial oxidation of H.sub.2 S by air, thereby forming a mixture of H.sub.2 S and SO.sub.2, and through the subsequent reaction of H.sub.2 S and SO.sub.2 to produce elemental sulphur and water vapor.
The partial oxidation of H.sub.2 S is frequently accomplished in Claus reaction furnaces using controlled amounts of air to obtain approximately a 2:1 H.sub.2 S/SO.sub.2 mixture in the resulting gases. Although a substantial part of the subsequent H.sub.2 S and SO.sub.2 reaction also frequently occurs in reaction furnaces, generally up to one-third of the H.sub.2 S and SO.sub.2 present in the reaction furnace must be transferred downstream for further catalytic processing.
In prior art processes, sulphur containing gases removed from the reaction furnaces are typically heated to a temperature above the sulphur dew point in order to avoid plugging the catalyst pores. At such temperatures, the rate of reaction is relatively slow and has to be repeated, generally three times, to achieve up to 98% (theoretical) conversion. In addition, the sulphur vapor produced in this reaction generally must be condensed after each step and reaction gases have to be reheated prior to contacting the next catalyst bed.
The remaining 2% of non-converted sulphur is usually removed in installations commonly known as tail gas clean up units. Such units operate using catalysts similar to those included in upstream catalyst converters, but at temperatures below the sulphur dew point in order to improve conversion. Catalyst plugging generally occurs in these units, which requires the use of multiple units and thus intermittent operation. Generally, multiple units are utilized such that when catalyst sulphur plugging reaches a predetermined value in one converter, a freshly regenerated converter is engaged in operation while the catalyst in the plugged converter is regenerated. Typically, three converters are used, one in production, one being regenerated, and one in post-regeneration cooling.
Although relatively high sulphur conversion can be achieved using the prior art combination processes, the use of multiple reaction beds results in a relatively high capital and energy cost as well as a complex flow pattern.
The problems suggested in the preceding are not intended to be exhaustive, but rather are among many which tend to reduce the effectiveness of prior art sulphur recovery systems. Other noteworthy problems may also exist, however, those presented above should be sufficient to demonstrate that such units appearing in the prior art have not been altogether satisfactory.